Semiconductor laser device

ABSTRACT

A semiconductor laser device having a waveguide constructed in a stack of layers including, on a substrate transparent and having a refractive index n s  for laser light, a first clad layer of a refractive index n c1 , a second clad layer of a refractive index n c2 , a third clad layer of a refractive index n c3 , a first conductivity type guide layer of a refractive index n g , an active quantum well layer, a second conductivity type guide layer, a second conductivity type clad layer, and a second conductivity type contact layer deposited in this order, wherein the waveguide has an effective refractive index n e , and a relationship of n c2 &lt;(n c1 , n c3 )&lt;n e &lt;(n s , n g ) is satisfied.

TECHNICAL FIELD

The present invention relates to a semiconductor laser device includinga substrate having a refractive index larger than an effectiverefractive index of an optical waveguide formed in the device. Theeffective refractive index of the optical waveguide is considered inanalyzing a guide mode in the waveguide. More specifically, the presentinvention relates to improvement in stability of a vertical transversemode in a semiconductor laser device including a plurality of nitridesemiconductor (In_(x)Al_(y)Ga_(1-x-y)N, where 0≦x, 0≦y, and x+y≦1)layers deposited on a GaN substrate for example, and also relates toprevention of cracks in a clad layer included in the semiconductorlayers.

For a definition to the effective refractive index of the waveguide,refer to a text book “HETEROSTRUCTURE LASERS” edited by Casey and Panishand published 1978 by ACADEMIC PRESS, pp. 42-49 (p. 49 in particular).

BACKGROUND ART

In recent years, blue light emitting diodes formed of nitridesemiconductor have been commercialized and furthermore blue laser diodeshave also been in practical utilization.

In FIG. 15, a nitride semiconductor laser device fabricated byconventional art on a GaN substrate is shown in a schematic front view.This laser device includes a 4 μm thick n-type GaN contact layer 802, ann-type In_(0.08)Ga_(0.92)N crack prevention layer 803, a 1.2 μm thickn-type AlGaN clad layer 804 (having a superlattice structure includingAl_(0.14)Ga_(0.86)N layers and GaN layers, and having a mixed-crystalcomposition of Al_(0.07)Ga_(0.93)N as averaged), a 0.075 μm thick n-typeGaN guide layer 805, an active quantum well layer 806 (including threepairs of an In_(0.11)Ga_(0.89)N well layer and an In_(0.01)Ga_(0.99)Nbarrier layer), a p-type Al_(0.4)Ga_(0.6)N electron trapping layer 807,a 0.075 μm thick p-type GaN guide layer 808, a 0.5 μm thick AlGaN cladlayer 809 (having a superlattice structure of Al_(0.1)Ga_(0.9)N layersand GaN layers, and having a mixed-crystal composition ofAl_(0.05)Ga_(0.95)N as averaged), and a 15 nm thick p-type GaN contactlayer 810, sequentially deposited on a GaN substrate 801.

FIG. 16 is a graph showing a radiation pattern in a direction verticalto the active layer (hereinafter referred to as a “vertical radiationpattern”) in a far field pattern (FFP) of optical radiation from theFIG. 15 laser device. More specifically, in this graph, a horizontalaxis represents the deviation angle (degrees) from a direction parallelto the active layer toward a direction perpendicular to the active layerand a vertical axis represents the optical intensity (a.u.: arbitraryunit). When the substrate is made of a material having a refractiveindex larger than an effective refractive index of a waveguide as in theFIG. 15 laser device, then in a transverse mode vertical to the activelayer (a vertical transverse mode), laser rays having reached thesubstrate are radiated through the substrate. As shown in FIG. 16,therefore, the FFP includes a noise peak at a direction deviated byabout ten and several degrees from an emission direction of afundamental mode (a direction parallel to the active layer) toward thesubstrate side (downward). A laser device causing such a noise peakinvolves a problem in application to optical disks etc. Further, such anoise peak corresponds to radiation loss of waveguide, involvingproblems of increase of threshold current in the laser device anddecrease of differential quantum efficiency in lasing.

On the other hand, when thick (4 μm) n-type Al_(0.05)Ga_(0.95)N contactlayer 802 is deposited between GaN substrate 801 and n-type AlGaN cladlayer 804 as in the FIG. 15 laser device, the radiation (or leakage)mode to GaN substrate 801 tends to be suppressed. In that case, however,n-type AlGaN clad layer 804 must be formed to have a relatively largethickness of approximately 0.8 μm, and it becomes difficult tocompletely prevent cracks in its crystal. As a consequence, electriccurrent leakage, increase of threshold current, and decrease ofreliability are involved, causing decrease of the laser device yieldrate.

Accordingly, a main object of the present invention is to suppress thenoise peak in the vertical radiation pattern in such a semiconductorlaser device including a substrate having a refractive index larger thanan effective refractive index of a waveguide as in the case that asemiconductor laser structure is fabricated on a GaN substrate. Anotherobject of the present invention is to prevent decrease of the nitridesemiconductor laser device yield rate attributed to cracks in the n-typeclad layer having a relatively large Al composition ratio, and tosuppress the noise peak in the vertical radiation pattern.

DISCLOSURE OF THE INVENTION

The present invention provides a semiconductor laser device having awaveguide formed in a stack of layers including a first clad layerhaving a refractive index n_(c1), a second clad layer having arefractive index n_(c2), a third clad layer having a refractive indexn_(c3), a first conductivity type (n-type or p-type) guide layer havinga refractive index n_(g), an active quantum well layer, a secondconductivity type (p-type or n-type) guide layer, a second conductivitytype clad layer, and a second conductivity type contact layersequentially deposited on a transparent substrate having a refractiveindex n_(s) for laser light, wherein the waveguide has an effectiverefractive index n_(e) satisfying a relationship: n_(c2)<(n_(c1),n_(c3))<n_(e)<(n_(s), n_(g)).

More specifically, according to the present invention, in the case ofusing a substrate having a refractive index ns larger than an effectiverefractive index ne in a waveguide region, a clad layer provided betweenthe substrate and an active layer is divided into at least three layersand among the three clad layers a clad layer having the smallestrefractive index is arranged between the other clad layers to reduceoptical radiation into the substrate. In particular, for a galliumnitride semiconductor laser device using a nitride semiconductorsubstrate such as a GaN substrate or an AlGaN substrate, the presentinvention can prevent cracks in a clad layer of Al containing nitridesemiconductor and can also suppress a radiation (or leakage) mode to thesubstrate, whereby significantly increasing the laser device yield rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of a semiconductor laser deviceaccording to a first embodiment of the present invention.

FIG. 2 is a graph showing an optical intensity profile of a verticalradiation pattern of the FIG. 1 semiconductor laser device.

FIG. 3 is a schematic front view of a semiconductor laser deviceaccording to a second embodiment of the present invention.

FIG. 4 is a graph showing an optical intensity profile of a verticalradiation pattern of a semiconductor device according to a fourthembodiment of the present invention.

FIG. 5 is a graph showing an optical intensity profile of a verticalradiation pattern of a semiconductor device according to a fifthembodiment of the present invention.

FIG. 6 is a schematic front view of a semiconductor laser deviceaccording to an eighth embodiment of the present invention.

FIG. 7 is a graph showing an optical intensity profile of a verticalradiation pattern of the FIG. 6 semiconductor laser device.

FIG. 8 is a schematic front view of a semiconductor laser deviceaccording to a ninth embodiment of the present invention.

FIG. 9 is a graph schematically showing an exemplary variation of Alcomposition ratio in a thickness direction of a first conductivity typeclad layer shown in FIG. 8.

FIG. 10 is a graph schematically showing another exemplary variation ofAl composition ratio in a thickness direction of a first conductivitytype clad layer in the ninth embodiment.

FIG. 11 is a graph schematically showing still another exemplaryvariation of Al composition ratio in a thickness direction of a firstconductivity type clad layer in the ninth embodiment.

FIG. 12 is a graph schematically showing still another exemplaryvariation of Al composition ratio in a thickness direction of a firstconductivity type clad layer in the ninth embodiment.

FIG. 13 is a schematic front view of a semiconductor laser deviceaccording to a tenth embodiment of the present invention.

FIG. 14 is a schematic front view for illustrating a radiation region ofa leakage mode and a portion immediately underlying a waveguide in thetenth embodiment.

FIG. 15 is a schematic front view of a semiconductor laser deviceprovided by conventional art.

FIG. 16 is a graph showing an optical intensity profile of a verticalradiation pattern of the semiconductor laser device provided by theconventional art.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention is applicable to a semiconductor laser deviceutilizing any material, which includes a transparent substrate having arefractive index larger than an effective refractive index of itswaveguide. More specifically, it is preferable to apply the presentinvention to a nitride semiconductor laser device including a GaN orAlGaN substrate, or an AlGaInAsP semiconductor laser device including aGaAs substrate, with a view to ensuring reliability of the device.

FIG. 1 shows, as one preferable application of the present invention, anitride semiconductor laser device including a nitride semiconductorsubstrate 101 and an active layer 107, between which a firstAl_(xc1)Ga_(1-xc1)N clad layer 103, a second Al_(xc2)Ga_(1-xc2)N cladlayer 104, a third Al_(xc3)Ga_(1-xc3)N clad layer 105 and a firstconductivity type guide layer 106 are stacked in this order from thesubstrate side. Note that among these semiconductor layers, at least onelayer may have the element N partially substituted with As, P, and/orSb. In this case, however, the element N desirably has its compositionratio of at least 0.9 in the group V elements, with a view to obtaininggood compositional uniformity and high crystal quality in the layer.Furthermore, any of the first to third clad layers may have In addedthereto at a composition ratio within a range of 0.01 to 0.05. In thiscase, the clad layer crystal can be grown at a lower temperature, andthen the grown crystal becomes softer, bringing about an effect offurther reducing the cracks.

While substrate 101 can be made of nitride semiconductor, it is mostpreferably made of GaN. In general, a GaN substrate has higher crystalquality as compared to other nitride semiconductor substrates and thusis preferably used to obtain a reliable semiconductor laser device. Inparticular, when the GaN substrate is made to have n-type conductivity,an electrode can be formed on a back surface of the substrate, which ispreferable with a view to reducing size of a semiconductor laser chip.AlGaN other than GaN can also be used as a material for the substrate.In this case, however, the Al composition ratio is desirebly at most0.02 in group III elements, with a view to preventing the substrate fromcracking.

Other than a substrate made only of nitride semiconductor, the presentinvention can also employ a pseudo nitride semiconductor substrateincluding a substrate of a different type and an overlying nitridesemiconductor layer. If for such a pseudo nitride semiconductorsubstrate, local crystal growth control films of SiO₂ or the like areused to utilize lateral growth of a nitride semiconductor layer toreduce dislocation density, i.e., epitaxial lateral over growth (ELOG)is employed, then the overlying nitride semiconductor layer contains thelocal films of SiO₂ or the like. Note that the present invention ispreferably applicable also to a laser device having a pseudo nitridesemiconductor substrate which includes a nitride semiconductor layerhaving a thickness of at least 10 μm and having a refractive indexlarger than an effective refractive index n_(e) of a waveguide formed inthe device, since such a laser device causes a significant noise peak inthe vertical radiation pattern as described above (see FIG. 16).

As shown in FIG. 1, an underlying layer 102 may be inserted betweenfirst clad layer 103 and substrate 101. Underlying layer 102 can be madeof n-type GaN. Underlying layer 102 of n-type GaN is preferred, since itcan reduce negative effects attributed to unevenness, scratches and thelike on the surface of GaN substrate 101 and can contribute to reducingcrystal defects in clad layer 103. Although underlying layer 102 canalternatively be made of n-type AlGaN, the Al composition ratio ofunderlying layer 102 is desirably at most 0.02 in the group IIIelements, with a view to reducing negative effects of unevenness,scratches and the like on the surface of substrate 101.

First clad layer 103 can be made of a nitride semiconductor having arefractive index smaller than effective refractive index n_(e) of thewaveguide. For example, first clad layer 103 can be made of n-typeAl_(xc1)Ga_(1-xc1)N, wherein the Al composition ratio x_(c1) is selectedso that the layer can have a refractive index smaller than n_(e). Morespecifically, when the Al ratio of the mixed-crystal in the AlGaNmaterial having a refractive index equal to n_(e) is represented byx_(ne), it is necessary to satisfy x_(ne)<x_(c1). On the other hand, itis preferable to satisfy x_(c1)≦0.07, with a view to reducing finecracks in the crystal of the first clad layer 103. More preferably, whenx_(c1)≦0.05 is satisfied, it is possible to reduce laser light radiationinto substrate 101 and reduce the vertical spread angle of FFP to 26° orless, and it becomes possible to increase the optical couplingcoefficient in coupling the semiconductor laser device with an opticalsystem.

Second clad layer 104 can be made of a nitride semiconductor having arefractive index smaller than that of first clad layer 103. For example,second clad layer 104 can be made of n-type Al_(xc2)Ga_(1-xc2)N, whereinthe Al composition ratio is selected to satisfy x_(ne)<x_(c1)<x_(c2),preferably in a range of 0.06≦x_(c2)≦0.3. More specifically, when the Alcomposition ratio x_(c2) is smaller than 0.06, the light amount trappedin active layer 107 is decreased, leading to increase in the thresholdcurrent of the laser device. On the other hand, when x_(c2) is largerthan 0.3, cracks are liable to occur in second clad layer 104 and thenthe laser device cannot be reliable.

Third clad layer 105 can be made of a nitride semiconductor having arefractive index larger than that of the second clad layer 104 andsmaller than effective refractive index n_(e) of the waveguide, wherebyincreasing effective refractive index n_(e) of the waveguide as comparedto the case that third clad layer 105 is not provided. Morespecifically, third clad layer 105 can be made of n-typeAl_(xc3)Ga_(1-xc3)N, wherein the Al composition ratio is selected tosatisfy x_(ne)<x_(c3)<x_(c2). On the other hand, it is preferable tosatisfy x_(c1)≦0.07, with a view to reducing fine cracks in the crystalof third clad layer 105.

When first, second and third clad layers 103, 104, 105 have thicknessesd_(c1), d_(c2), d_(c3), respectively, with d_(c2)<d_(c1), andd_(c3)<d_(c1) being set, the leakage mode into substrate 101 caneffectively be prevented and the second clad layer having the largest Alcomposition ratio can have thickness d_(c2) reduced so as to reduce thecracks. Second clad layer 104 preferably has a thickness in a range of0.05 μm≦d_(c2)≦0.35 μm. In the case of 0.05>d_(c2), second clad layer104 loses contribution to the light trapping effect, leading to increaseof the threshold current. In the case of d_(c2)>0.35 μm, on the otherhand, the FFP's vertical full angle at half maximum is increased to belarger than 26°, leading to reduction of the optical couplingcoefficient in coupling the laser device with a lens. Furthermore, inthe case of d_(c2)>0.35 μm, crystal cracks are liable to occur in thethree clad layers, leading to reduction in the laser device yield rate.

Three clad layers 103, 104, 105 together preferably have a totalthickness d_(t)=d_(c1)+d_(c2)+d_(c3) of 4.5 μm or less. In the case oftotal thickness d, exceeding 4.5 μm, even if the cracks are suppresseduntil all semiconductor layers are grown on the substrate, subsequentheat treatments for activation of p-type impurities and alloying of anelectrode cause the crystal cracks. In the case that total thicknessd_(t) is smaller than 1.4 μm, on the other hand, the effect ofsuppressing the leakage mode into substrate 101 is reduced, causinglaser loss.

First, second and third clad layers 103, 104, 105 are not restrictedregarding their conductivity type. In the case that at least one of thelayers is of p-type conductivity or insulative, however, an n-typeelectrode must be formed on a surface of an n-type layer existing closerto active layer 107 than the p-type or insulative clad layer. In thecase that at least one clad layer is an undoped layer, on the otherhand, it is possible to reduce laser absorption loss attributed toabsorption caused by free carriers. In the case that three clad layers103, 104, 105 are all made of n-type semiconductor, the n-type electrodecan be formed on substrate 101, whereby making it possible to form theelectrode through a simplified process and reduce contact resistance ofthe electrode. Furthermore, by making substrate 101 as well as the threeclad layers have n-type conductivity, it becomes possible to form then-type electrode on a back surface of substrate 101, whereby making itpossible to reduce size of a laser chip and package the chip through asimplified process.

It is not necessary that three clad layers 103, 104, 105 are in contactwith each other. A thin layer of InGaN, GaN or InGaAIN may be insertedbetween first and second clad layers 103 and 104 or between second andthird clad layers 104 and 105. In the case that such a thin layer isinserted, the thin layer desirably has a thickness (corresponding to adistance between the first and second clad layers or between the secondand third clad layers) set to be one fourth or less of a wavelength oflaser light in the waveguide and desirably has a thickness of 0.04 μm orless for a bluish violet laser of nitride semiconductor.

Furthermore, a strain alleviation layer of InGaN (not shown) may beinserted between first clad layer 103 and nitride semiconductorsubstrate 101 or underlying layer 102. The strain alleviation layer canhave a thickness selected in a range of 0.02 μm to 0.06 μm. If thethickness is smaller than 0.02 lm, the layer can not cause the effect ofalleviating strain. If the thickness is larger than 0.06 μm, on theother hand, the strain alleviation layer is liable to include pitsformed therein and then semiconductor layers grown thereon are liable tohave impaired crystallinity. Furthermore, the strain alleviation layerdesirably has an In composition ratio of at least 0.03 and at most 0.12.If the In composition ratio is smaller than 0.03, the layer can notcause the effect of alleviating strain. If the In composition ratio islarger than 0.12, the layer is liable to include pits formed therein andthen the semiconductor layers grown thereon are liable to have impairedcrystallinity. It is preferable to introduce the strain alleviationlayer, because the layer can significantly cause the effect ofalleviating strain when the first to third clad layers deposited thereonhave (d_(c1)×x_(c1)+d_(c2)×x_(c2)+d_(c3)×x_(c3)) exceeding 0.15 μm.

Furthermore, at least one of the first, second and third clad layers mayhave a multi-layer structure. In a specific example of the multi-layerstructure, an AlGaN layer having a thickness of 0.1 μm and an Alcomposition ratio of 0.1 can be replaced with a 20 nm thickAl_(0.2)Ga_(0.8)N layer and a 20 nm thick GaN layer alternately repeated25 times. Although it is possible to select an Al composition ratio, athickness and the like as desired for each layer in the multi-layerstructure, when each layer has a thickness d_(i) and an Al compositionratio xi, the average Al composition ratio of Σ(d_(i)·x_(i))/Σ(d_(i))must satisfies the above described condition for the clad layer.

First conductivity type guide layer 106 can be made of a nitridesemiconductor having a refractive index larger than the waveguide'seffective refractive index n_(e). For example, guide layer 106 can bemade of n-type GaN or n-type InGaN, and it preferably has a thickness ofat least 0.03 μm and at most 0.2 μm. When the thickness of guide layer106 is smaller than 0.03 μm or larger than 0.2 μm, the effect oftrapping light in active layer 107 is reduced and the threshold currentof the laser device is increased. Furthermore, when n-type guide layer106 is made with an InGaN material, it is preferable to select its Incomposition ratio in a range of 0.01 to 0.1. In the case of the Incomposition ratio exceeding 0.1, efficiency of electron injection intoactive layer 107 is reduced and the laser device's threshold current isincreased.

Active layer 107 can have a single quantum well structure formed ofInGaN or a multiple quantum well structure including more than one pairof a quantum well layer of InGaN and a barrier layer of GaN, InGaN, orAlGaInN. In particular, by utilizing the multiple quantum well structureincluding the barrier layer of InGaN, light amount trapped in activelayer 107 can be increased and the laser device's threshold current canbe reduced. Furthermore, by selecting the number of quantum wells in arange of 2 to 5 in active multiple quantum well layer 107, the laserdevice's threshold current can be reduced. Furthermore, when activemultiple quantum well layer 107 has a total thickness of at least 0.04μm and at most 0.08 μm, the laser device's threshold current can bereduced. Furthermore, when active layer 107 has a thickness of at least0.04 μm and at most 0.06 μm, it is possible to make the vertical laserradiation angle smaller than 24°. Active layer 107 may be doped with Si,Sn, Se, Te or the like. In particular, when active layer 107 is dopedwith Si in a range of 1×10¹⁷ cm⁻³ to 5×10¹⁸ cm⁻³, it is possible tomaximize the differential gain and reduce the threshold current in thelaser device.

A plurality of semiconductor layers over active layer 107 need to form astacked layer structure which can simultaneously implement a heterostructure trapping carriers in active layer 107 and a waveguidestructure trapping light in active layer 107. For example, a p-typeprotection layer 108, a p-type guide layer 109, a p-type clad layer 110,and a p-type contact layer 111 deposited in this order over active layer107, and then a ridged stripe structure is formed with at least p-typeclad layer 110 and p-type contact layer 111.

P-type protection layer 108 can be made of a nitride semiconductorhaving a barrier height of its conduction band higher by at least 0.3 eVas compared to active layer 107. For example, p-type protection layer108 can be made of p-type AlGaN doped with Mg, and it preferably has anAl composition ratio of at least 0.1 and at most 0.45. When the Alcomposition ratio is smaller than 0.1, the barrier height for electronsis reduced, so that electrons cannot effectively be trapped in activelayer 107 and overflow into p-type layers, thereby increasing the laserdevice's threshold current. In the case of the Al composition ratioexceeding 0.45, on the other hand, it becomes difficult to activate theadded Mg, so that the effective barrier height for electrons is reduced.More preferably, an Al composition ratio of at least 0.3 and at most0.45 can be selected to ensure that the laser device reliably operatesat a high temperature (of 70° C. or more). P-type protection layer 108preferably has a thickness of at least 5 nm and at most 30 nm. When thethickness is smaller than 5 nm, the layer loses uniformity and partiallyallows an overflow of electrons from active layer 107 to p-type layers.When the thickness is larger than 30 nm, on the other hand, protectionlayer 108 gives an increased crystal strain effect to active layer 107and the laser device's threshold current is increased.

Second conductivity type guide layer 109 can be made of a nitridesemiconductor having a refractive index larger than n_(e). For example,guide layer 109 can be made of Mg added p-type GaN, InGaN or InGaAlN,and it preferably has a thickness of at least 0.03 μm and at most 0.2μm. When the thickness is smaller than 0.03 μm or larger than 0.2 μm,the effect of trapping light in active layer 107 is reduced and thethreshold current of the laser device is increased. Furthermore, whenInGaN or InGaAlN is used for second conductivity type guide layer 109,it is preferable to select its In composition ratio in a range of 0.01to 0.1. In the case of the In composition ratio exceeding 0.1,efficiency of electron injection into active layer 107 is reduced andthe laser device's threshold current is increased.

Second conductivity type clad layer 110 can be made of a nitridesemiconductor having a refractive index smaller than n. For example,clad layer 110 can be made of Mg added p-type AlGaN and it preferablyhas an Al composition ratio of at least 0.06 and at most 0.2. When theAl composition ratio is smaller than 0.06, active layer 107 lesseffectively traps light and the laser device's threshold current isincreased. In the case of the Al composition ratio exceeding 0.2, p-typeconductance due to the addition of Mg is reduced and the laser device'slifetime is reduced. P-type clad layer 109 preferably has a thickness ofat least 0.25 μm and at most 1.5 μm. When the thickness is smaller than0.25 μm, a significant part of laser light is absorbed by a p-typeelectrode 112, leading to increase loss at the waveguide. When thethickness is larger than 1.5 μm, the clad layer's resistance isincreased and the laser device's lifetime is reduced. More preferably,the thickness can be selected in a range of 0.35 μm to 0.75 μm to reducethe cracks and also make it possible to form a ridged stripe of a widthas small as 1 to 2 μm.

Second conductivity type contact layer 111 is made of a nitridesemiconductor that can establish an ohmic contact with secondconductivity type electrode 112. For example, contact layer 111 can bemade of Mg added p-type GaN and can also be made of a material includingIn added thereto in a composition ratio range of 0.01 to 0.15. Withcontact layer 111 of such a material, it is possible to increase holedensity as compared to that of GaN, whereby making it possible to reducecontact resistance of second conductivity type electrode 112. Secondconductivity type contact layer 111 preferably has a thickness of atleast 0.06 μm and at most 0.2 μm. When the thickness is smaller than0.06, the layer can no longer serve as a contact layer to establishlow-resistance contact with second conductivity type electrode 112. Incontrast, when second conductivity type contact layer 111 having arefractive index larger than ne has a thickness larger than 0.2 μm, thenin addition to the waveguide with active layer 107 serving as a core, asecondary waveguide with contact layer 111 serving as a core is formed,so that laser light is coupled with the secondary waveguide too therebycausing output loss.

Before a semiconductor stacked-layer structure is formed on nitridesemiconductor substrate 101, the substrate may have its surface etched.The nitride semiconductor substrate sometimes includes unevenness on itssurface, depending on the method of forming the same. In such a case, byetching the substrate surface to make it flat and then depositingunderlying layer 102, first clad layer 103 and the like thereon, itbecomes possible to improve crystallinity of the semiconductorstacked-layer structure.

In the present invention, crystal growth of nitride semiconductor layerscan be carried out by metal-organic vapor phase epitaxy (MOVPE),metal-organic chemical vapor deposition (MOCVD), halide vapor phaseepitaxy (HVPE), molecular beam epitaxy (MBE) or any other similartechnique that can grow the nitride semiconductor layers.

First Embodiment

As a first embodiment, a process of fabricating the FIG. 1 nitridesemiconductor laser device will be described. Initially, n-type GaNunderlying layer 102 of 3 μm thickness is grown on n-type GaN substrate101 at 1125° C., using hydrogen as a carrier gas, trymethylgallium (TMG)and ammonium as source gases, and silane (SiH₄) as a dopant gas.

Then, at the same substrate temperature, hydrogen serving as a carriergas, trymethy aluminum (TMA), TMG and ammonium serving as source gases,and silane serving as a dopant gas are used to grow first n-type cladlayer 103 of Al_(0.05)Ga_(0.95)N doped with Si at a concentration of3×10¹⁸ cm⁻³ to have a thickness of 1.8 μm. Similarly, second n-type cladlayer 104 of Al_(0.1)Ga_(0.9)N doped with Si at a concentration of3×10¹⁸ cm⁻³ is grown to a thickness of 0.2 μm. Similarly, third n-typeclad layer 105 of Al_(0.05)Ga_(0.95)N doped with Si at a concentrationof 3×10¹⁸ cm⁻³ is grown to a thickness of 0.1 μm.

Then, at the same substrate temperature, hydrogen serving as a carriergas, TMG and ammonium serving as source gases, and silane serving as adopant gas are used to grow n-type guide layer 106 of GaN doped with Siat a concentration of 8×10¹⁷ cm⁻³ to have a thickness of 0.08 μm.

Then, at the substrate temperature reduced to 760° C., and nitrogen orargon serving as a carrier gas, TMG and ammonium serving as sourcegases, and silane serving as a dopant gas are used to grow a barrierlayer of GaN doped with Si at a concentration of 2×10¹⁸ cm⁻³ to have athickness of 12 nm. Subsequently, the silane gas is stopped and TMG isintroduced to grow an undoped In_(0.11)Ga_(0.89)N well layer to athickness of 4 nm. These barrier and well layers are repeatedly formedthree times and thereafter a final barrier layer is deposited tocomplete active layer 107 having a multiple quantum well structure (MQW)of 60 nm thickness in total.

Then, at the same substrate temperature, hydrogen serving as a carriergas, TMA, TMG and ammonium serving as source gases, and furthercyclopentadienylmagnesium (Cp₂Mg) serving as a dopant gas are used togrow p-type protection layer 108 of Al_(0.4)Ga_(0.6)N containing Mg at aconcentration of 1×10¹⁹ cm⁻³ to have a thickness of 20 nm.

Then, at the substrate temperature raised to 1035° C., nitrogen servingas a carrier gas, TMG and ammonium serving as source gases, and Cp₂Mgserving as a dopant gas are used to grow p-type guide layer 109 of GaNcontaining Mg at a concentration of 2×10¹⁹ cm⁻³ to have a thickness of0.08 μm.

Then, at the same substrate temperature, nitrogen serving as a carriergas, and TMA, TMG and ammonium serving as source gases are used to growan undoped Al_(0.1)Ga_(0.09)N layer A to a thickness of 2.5 nm and thenTMA is stopped and Cp₂Mg is used as a dopant gas to grow a layer B ofGaN doped with Mg at a concentration of 3×10¹⁹ cm⁻³ to have a thicknessof 2.5 nm. Layers A and B are alternately deposited repeatedly 100 timesto complete p-type clad layer 110 including a multilayer film (asuperlattice structure) having a total thickness of 0.5 μm.

Then, at the same substrate temperature, nitrogen serving as a carriergas, TMG and ammonium serving as source gases, and Cp₂Mg serving as adopant gas are used to grow p-type contact layer 111 of GaN doped withMg at a concentration of 1.5×10²⁰ cm⁻³ to have a thickness of 60 nm.

A wafer including a plurality of semiconductor layers thus grown iscooled approximately to a room temperature and taken out from thedeposition chamber, and then p-type electrode layer 112 made ofpalladium/molybdenum/gold is deposited on p-type contact layer 111.Then, a resist mask is formed in a stripe (not shown) on p-typeelectrode layer 112 and reactive ion etching (RE) is utilized to form aridged stripe 114. More specifically, Ar gas is used to etch p-typeelectrode 112 and then a gaseous mixture of Ar, Cl₂ and SiCl₄ is used toetch from p-type contact layer 111 to some extent of p-type clad layer110 or to some extent of p-type guide layer 109, thereby forming ridgedstripe 114 having a bottom side of 1.6 μm width and a top surface of 1.3μm width on p-type electrode 112.

Furthermore, with the resist being left on ridged stripe 114, thewafer's top surface is covered with an insulating film 115 (herein, a Zroxide mainly made of ZrO₂) grown to a thickness of 0.5 μm. Thereafter,the resist is removed to expose the top side of ridged stripe 114.

Then, n-type GaN substrate 101 has its back surface ground and polishedto have a thickness of 110 μm. Thereafter, an n-type electrode 113 isformed on the back surface of the substrate and alloying of theelectrode is carried out at 530° C. for about 2 minutes. Then, a p-typepad electrode 116 made of molybdenum and gold is formed to cover p-typeelectrode 112 and insulating film 115 at least in the vicinity of eitherside of the electrode. Then, broken-line scribing is employed to formcavity's end surfaces with cleavage planes of the wafer and thereafterchip division is carried out to produce semiconductor laser devices.Incidentally, the cavity preferably has a length in a range of 180 to850 μm.

The obtained laser device is die-bonded to a heat sink and p-type padelectrode 116 is wire-bonded. The laser devise is energized to lase at aroom temperature, and continuous lasing is confirmed with a thresholdcurrent of 2.5 kAcm⁻², a threshold voltage of 4.3V and an emissionwavelength of 405 nm. Furthermore, the laser device having its lifetimeof at least 30,000 hours at 70° C. is obtained with a yield rate of 83%.

FIG. 2 shows an optical intensity profile of a vertical radiationpattern in an FFP of the laser device of the first embodiment. In FIG.2, such a noise peak as shown in FIG. 16 attributed to a conventionalleakage mode is reduced to a significant extent which does not cause aproblem when the laser device is applied to an optical disk apparatusfor example. Incidentally, the plurality of semiconductor layers in thefirst embodiment do not include observable cracks. Furthermore, the fullangle at half maximum of the vertical radiation pattern in the FFP inthe first embodiment can be reduced to 22.5° (see FIG. 2), and thus ithas a small ratio of 2.1 relative to 10.5° of the full angle at halfmaximum of the horizontal radiation pattern in the same FFP. With thelaser device of the first embodiment, therefore, it is possible toimprove efficiency of using laser light in collecting the laser light bya lens.

Second Embodiment

FIG. 3 is a schematic front view of a laser device according to a secondembodiment. As seen from FIG. 3 in comparison with FIG. 1, the secondembodiment provides a laser device different from the first embodimentonly in that a strain alleviation layer 120 is inserted between n-typeunderlying layer 102 and first n-type clad layer 103. Strain alleviationlayer 120 can be made of n-type In_(0.09)Ga_(0.91)N for example.

More specifically, at a substrate temperature of 800° C., a gaseousmixture of nitrogen of 95% and hydrogen of 5% serving as a carrier gas,TMG, TMI and ammonium serving as source gases, and further, silaneserving as a dopant gas can be used to form n-type In_(0.09)Ga_(0.91)Nstrain alleviation layer 120 doped with Si at a concentration of 5×10¹⁸cm⁻³ to have a thickness of 0.03 μm.

The second embodiment provides an increased yield rate of 94% of thelaser device which shows similar characteristics as in the firstembodiment.

Third Embodiment

A third embodiment provides a laser device different from the firstembodiment only in that dielectric layer 115 shown in FIG. 1 is replacedwith a high-resistance or n-type AlGaN layer. Such a laser device of thethird embodiment can bring about similar effects as in the firstembodiment. The AlGaN layer can be deposited for example by MOVPE,MOCVD, HVPE, MBE, or other similar crystal growth technique.Furthermore, if sputtering is employed to deposit the AlGaN layer, thesubstrate's temperature can be set to 700° C. or less, which is lowerthan that for crystal growth, and this is preferable with a view topreventing thermal degradation of active layer 107.

Furthermore, the AlGaN layer preferably has an Al composition ratiohigher than that of p-type clad layer 110 so as to realize a steadytransverse mode up to a high output. Furthermore, if the Al compositionratios of the AlGaN layer and p-type clad layer 110 are made equal andthe etching for forming ridged strip 114 is carried out to reach p-typeguide layer 109, it is possible to realize a steady transverse mode upto a high output and effectively prevent leak current associated withcracks in the AlGaN layer.

Fourth Embodiment

A fourth embodiment provides a laser device different from the firstembodiment only in that x_(c1)=0.038 and d_(c1)=3.3 μm are set in theconditions regarding first clad layer 103. FIG. 4 shows a verticalradiation pattern in an FFP of the laser device of the fourthembodiment. As seen in FIG. 4, the present embodiment substantiallycompletely prevents a noise peak in the vertical radiation pattern inthe FFP (see also FIG. 16), with the full angle at half maximum of thevertical radiation pattern being as small as 22°, thereby making itpossible to obtain the most preferable laser device for its application.

Fifth Embodiment

A fifth embodiment provides a laser device different from the firstembodiment only in that x_(c2)=0.07 and d_(c2)=0.35 μm are set in theconditions regarding second clad layer 104. FIG. 5 shows a verticalvariation pattern in an FFP of the laser device of the fifth embodiment.As see in FIG. 5, the present embodiment can also significantly preventa noise peak in the vertical radiation pattern, with the full angle athalf maximum of the vertical radiation pattern being as small as 23°,thereby making it possible to obtain a satisfactory laser device.

Sixth Embodiment

A sixth embodiment provides a laser device different from the firstembodiment only in that x_(c3)=0.05 and d_(c3)=0.05 μm are set in theconditions regarding third clad layer 105. The sixth embodiment alsobrings about similar effects as in the fifth embodiment.

Seventh Embodiment

A seventh embodiment provides a laser device different from the firstembodiment only in that active quantum well layer 107 is slightlymodified. More specifically, the active layer in the seventh embodimentis formed as follows: after n-type guide layer 106 is deposited, thesubstrate's temperature is set to 800° C., and nitrogen serving as acarrier gas, TMI, TMG and ammonium serving as source gases, and furthersilane serving as a dopant gas are used to grow a barrier layer ofIn_(0.01)Ga_(0.99)N doped with Si at a concentration of 5×10¹⁷ cm⁻³ tohave a thickness of 8 nm. Subsequently the silane gas is stopped, and awell layer of undoped In_(0.11)Ga_(0.89)N is grown to a thickness of 3nm. These barrier and well layers are deposited repeatedly five timesand thereafter a final barrier layer is grown to complete active layer107 including a multiple quantum well structure (MQW) having a totalthickness of 63 nm. The seventh embodiment also brings about similareffects as in the fifth embodiment.

Eighth Embodiment

FIG. 6 is a schematic front view of an AlGaAs laser device according toan eighth embodiment. This laser device is fabricated as follows:initially, on an n-type GaAs substrate 601 at a temperature of 720° C.,hydrogen serving as a carrier gas, TMG and arsine serving as sourcegases, and silane serving as a dopant gas are used to grow an n-typeGaAs underlying layer 602 to a thickness of 0.5 μm.

Then, at the same substrate temperature, hydrogen serving as a carriergas, TMA, TMG and arsine serving as source gases and furthermore,serving silane as a dopant gas are used to grow a first n-type cladlayer 603 of Al_(0.04)Ga_(0.96)As doped with Si at a concentration of3×10¹⁸ cm⁻³ to have a thickness of 3.5 μm. Similarly, a second n-typeclad layer 604 of Al_(0.5)Ga_(0.8)As doped with Si at a concentration of2×10¹⁸ cm⁻³ is grown to a thickness of 0.15 μm, and a third n-type cladlayer 605 of Al_(0.05)Ga_(0.96)As doped with Si at a concentration of1×10 ¹⁸ cm⁻³ is grown to a thickness of 0.1 μm. Then, TMA is stopped andan n-type guide layer 606 of GaAs doped with Si at a concentration of8×10¹⁷ cm⁻³ is grown to a thickness of 0.12 μm.

Then, at the substrate temperature reduced to 680° C., hydrogen servingas carrier gas, and TMG and arsine serving as source gases are used togrow a barrier layer of undoped GaAs to a thickness of 20 nm. Then, TMAis additionally supplied to grow a well layer of undopedIn_(0.09)Ga_(0.91)As to a thickness of 11 nm. These barrier and welllayers are repeatedly deposited twice and a final barrier layer is thengrown to complete an active layer 607 including a multiple quantum wellstructure (MQW) having a total thickness of 82 nm.

Then, at the same substrate temperature, hydrogen serving as a carriergas, TMA, TMG and arsine serving as source gases, and furtherdiethylzinc (DEZn) serving as a dopant gas are used to grow a p-typeprotection layer 608 of Al_(0.2)Ga_(0.8)As doped with Zn at aconcentration of 1.3×10¹⁸ cm⁻³ to have a thickness of 20 nm.

Then, at the substrate temperature raised to 720° C., hydrogen servingas a carrier gas, TMG and arsine serving as source gases, and DEZnserving as a dopant gas are used to grow a p-type guide layer 609 ofGaAs doped with Zn at a concentration of 7×10 ¹⁷ cm⁻³ to have athickness of 0.12 μm. Then, at the same substrate temperature, hydrogenserving as a carrier gas, TMA, TMG and arsine serving as sources gases,and DEZn serving as a dopant gas are used to grow a p-type clad layer610 of Al_(0.2)Ga_(0.8)As doped with Zn at a concentration of 1.6×10¹⁸cm⁻³ to have a thickness of 1.5 μm.

Then, at the same substrate temperature, nitrogen serving as a carriedgas, TMG and ammonium serving as source gases, and DEZn serving as adopant gas are used to grow a p-contact layer 611 of GaAs doped with Znat a concentration of 3×10¹⁸ cm⁻³ to have a thickness of 1.0 μm.

A wafer including a plurality of semiconductor layers thus grown iscooled approximately to a room temperature and taken out from thedeposition chamber. On the obtained wafer, a silicon oxide film in theform of a stripe (not shown) is used as a mask and RIE is employed toform a ridged stripe. More specifically, Cl₂ gas is used to etch fromp-type contact layer 611 to some extent of p-type clad layer 610 or tosome extent of p-type guide layer 609, thereby forming a ridged stripe614 having a top stripe-width of 3 μm on the upper surface of p-typeelectrode and a bottom stripe-width of 4 μm.

Then, with the mask of silicon oxide film held, the wafer is again setin a crystal growth apparatus. At a substrate temperature of 720° C.,hydrogen serving as a carrier gas, TMG and arsine serving as sourcegases, and silane serving as a dopant gas are used to selectively growan n-type GaAs current constriction layer 615 of 1.0 μm thickness.

After the wafer is cooled approximately to a room temperature, thesilicon oxide film used as the mask for the selective growth is removedand then a p-type electrode 612 of zinc/gold is formed on exposed p-typecontact layer 611 at the top of ridged stripe 614.

Then, n-type GaAs substrate 601 has its back surface ground and polishedto have a thickness of 90 μm and thereafter an n-type electrode 613 isformed on the back surface. Then, alloying of the electrode is carriedout in a vacuum at 450° C. for three minutes. Then, a p-type padelectrode 616 of molybdenum and gold is formed to cover p-type electrode612 and current constriction layer 615 at least in the vicinity of bothside of electrode 612. Finally, the wafer is cleaved to form endsurfaces of a cavity and then divided into chips to producesemiconductor laser devices. Incidentally, the cavity preferably has alength in a range of 180 to 850 μm.

The obtained laser device is die-bonded to a heat sink and p-type padelectrode 616 is wire-bonded. The laser devise is energized to lase at aroom temperature, and continuous lasing is confirmed with a thresholdcurrent of 450 Acm⁻², a threshold voltage of 1.7V and an emissionwavelength of 895 nm. Furthermore, the laser device having its lifetimeof at least 20,000 hours at 85° C. is obtained with a yield rate of 80%.

FIG. 7 shows a vertical radiation pattern in an FFP of the laser deviceprovided in the eighth embodiment. As seen in FIG. 7, the eighthembodiment substantially completely eliminates such a conventional noisepeak as shown in FIG. 16. Furthermore in the present embodiment, thefull angle at half maximum of the vertical radiation pattern in the FFPcan be reduced to 23°, and thus it has a small ratio of 2.3 relative to10° of the full angle at half maximum of the horizontal radiationpattern in the same FFP. With the laser device of the eighth embodiment,therefore, it is possible to improve efficiency of using laser light incollecting the laser light by a lens.

Ninth Embodiment

FIG. 8 is a schematic front view of a laser device provided by a ninthembodiment. This laser device includes n-type clad layers whoserefractive indexes are continuously changed in the thickness direction.This laser device is fabricated as follows: initially, on an n-type GaNsubstrate 701 at 1125° C., hydrogen serving as a carrier gas, TMG andammonium serving as source gases, and silane serving as a dopant gas areused to grow an n-type GaN underlying layer 702 to a thickness of 3 μm.

Then, at the same substrate temperature, hydrogen serving as a carriergas, TMA, TMG and ammonium serving as source gases, and silane servingas a dopant gas are used to grow a first n-type clad layer 703 ofAl_(x)Ga_(1-x)N doped with Si at a concentration of 3×10¹⁸ cm⁻³ to havea thickness of 2.5 μm. Herein, first n-type clad layer 703 has an Alcomposition ratio x equal to zero at the lower interface andmonotonously increased to 0.12 at the upper interface. Note that whileAl composition ratio x is linearly changed to increase in proportion tothe layer's thickness in the ninth embodiment, x may be increased as aquadratic of the layer's thickness or may be increased stepwise.

Then, at the same substrate temperature, with a second clad layer beingomitted, the same types of gases are used to grow a third n-typeAl_(y)Ga_(1-y)N clad layer 705 doped with Si at a concentration of3×10¹⁸ cm⁻³ to have a thickness of 0.3 μm. Herein, third n-type cladlayer 705 has an Al composition ratio y equal to 0.12 at the lowerinterface (the interface with first n-type clad layer 703) andmonotonously decreased to zero at the upper interface.

FIG. 9 is a graph showing a variation of the Al composition ratio in thethickness direction of n-type clad layers 703 and 705 in the ninthembodiment. In this graph, a horizontal axis represents the thickness(micrometers) from the bottom surface of n-type clad layer 703 and avertical axis represents the Al composition ratio. Note that first andthird n-type clad layers 703, 705 have their respective Al compositionratios continuously changed to a maximum value at a region in thevicinity of the interface between first and third n-type clad layers703, 705. Since that region functions as a second n-type clad layer, itis not necessary to form another layer serving as the second clad layer.In the ninth embodiment, therefore, it is important that n-type cladlayers 703, 705 are formed to provide at least three regions havinghigh, low and high refractive indexes, respectively, in a direction fromsubstrate 701 toward an active layer 707.

Furthermore, at the same substrate temperature, hydrogen serving as acarrier gas, TMG and ammonium serving as source gases, and silaneserving as a dopant gas are used to grow an n-type guide layer 706 ofGaN doped with Si at a concentration of 8×10¹⁷ cm⁻³ to have a thicknessof 0.13 μm.

Then, at the substrate temperature reduced to 780° C., nitrogen or argonserving as a carrier gas, TMG and ammonium serving as sources gases, andsilane serving as a dopant gas are used to grow a barrier layer of GaNdoped with Si at a concentration in a range of 1×10¹⁹ to 3×10²⁰ cm⁻³ tohave a thickness of 10 nm. Then, the silane is stopped and TMI isadditionally introduced to grow a well layer of undopedIn_(0.1)Ga_(0.9)N to a thickness of 3 nm. These barrier and well layersare repeatedly deposited three times and thereafter a final barrierlayer is grown to complete active layer 707 including a multiple quantumwell structure (MQW) having a total thickness of 49 nm.

Then, at the same substrate temperature, silane, TMG and ammonium areused to deposit an n-type GaN band adjustment layer 720 to a thicknessof 7 nm. Furthermore, TMG and ammonium are used to deposit an undopedGaN intermediate layer 721 to a thickness of 70 nm. Then, hydrogenserving as a carrier gas, TMA, TMG and ammonium serving as source gases,and Cp₂Mg serving as a dopant gas are used to grow a p-type protectionlayer 708 of Al_(0.15)Ga_(0.85)N doped with Mg at a concentration of2×10²⁰ cm⁻³ to have a thickness of 12 nm.

Furthermore, at the substrate temperature raised to 1035° C., nitrogenserving as a carrier gas, TMG and ammonium serving as source gases, andCp₂Mg serving as a dopant gas are used to grow a p-type guide layer 709of GaN doped with Mg at a concentration of 2×10¹⁹ cm⁻³ to have athickness of 0.08 μm. Then, at the same substrate temperature, nitrogenserving as a carrier gas, TMA, TMG and ammonium serving as sourcesgases, and further CP₂Mg serving as a dopant gas are used to grow ap-type A_(0.07)Ga₀₉₃N clad layer 710 doped with Mg at a concentration of9×10¹⁹ cm⁻³ to have a thickness of 0.5 μm. Furthermore, at the samesubstrate temperature, nitrogen serving as a carrier gas, TMG andammonium serving as source gases, and Cp₂Mg serving as a dopant gas areused to grow a p-type contact layer 711 of GaN doped with Mg at aconcentration of 1.5×10²⁰ cm⁻³ to have a thickness of 60 nm.

A wafer including a plurality of semiconductor layers thus grown iscooled approximately to a room temperature and taken out from thedeposition chamber. A p-type electrode layer 712 ofpalladium/molybdenum/gold is formed so as to cover p-type contact layer711 of the wafer. Then, a resist mask (not shown) is formed on p-typeelectrode 712 and RIE is employed to form a ridged stripe. Morespecifically, Ar gas is used to etch p-type electrode 712 andfurthermore a gaseous mixture of Ar, Cl₂ and SiCl₄ is used to etch fromp-type contact layer 711 to same extent of p-type clad layer 710 or tosome extent of p-type guide layer 709, thereby forming a ridged stripe714 having a top stripe-width of 1.6 μm on the upper surface of p-typeelectrode 712 and a bottom stripe-width of 1.8 μm.

Furthermore, with ridged stripe 714 having the resist left thereon, thewafer's top surface is covered with an insulating film 715 (herein,silicon oxide) grown to a thickness of 0.2 μm. Thereafter, the resist isremoved to expose the top surface of ridged stripe 714.

Then, n-type GaN substrate 701 has its back surface ground and polishedto have a thickness of 130 μm. Thereafter, an n-type electrode 713 ofmetal containing hafnium and aluminum is formed on the back surface ofthe substrate. Then, alloying of the electrode is carried out at 480° C.for about two minutes. Then, a p-type pad electrode 716 made ofmolybdenum and gold is formed to cover p-type electrode 712 overlyingridged stripe 714 as well as insulating film 715 at least in thevicinity of either side of the electrode. Finally, broken-line scribingis employed to cleave the wafer to form cavity's end surfaces andthereafter it is divided into chips to produce semiconductor laserdevices. Incidentally, the cavity preferably has a length in a range of180 to 850 μm.

The obtained laser device is die-bonded to a heat sink and p-type padelectrode 716 is wire-bonded. The laser devise is energized to lase at aroom temperature, and continuous lasing is confirmed with a thresholdcurrent of 2.1 kAcm⁻², a threshold voltage of 4.2V and an emissionwavelength of 400 nm. Furthermore, the laser device has its lifetime ofat least 20,000 hours at 80° C.

FIG. 9 shows a profile of a variation in Al composition ratio in thethickness direction of n-type clad layers 703 and 705. By graduallychanging the Al composition ratios of the n-type clad layers in thismanner, cracking in the AlGaN layer can more effectively be prevented ascompared to the first embodiment, and the yield rate of the laser devicecan be increased.

The full width at half maximum of a vertical radiation pattern in an FFPof the laser device of the ninth embodiment is as small as 16°, and itis possible to effectively suppress the noise peak attributed to lightradiated into substrate 701. Furthermore, the laser device of the ninthembodiment gives 10.5° for the full width at half maximum of thehorizontal radiation pattern in the FFP, and thus can provide theellipticity as small as 1.5. Therefore, the present embodiment's laserdevice can effectively be used in a pickup for a optical disk, therebyomitting a beam shaping prism to allow miniaturization and production atreduced costs.

In the ninth embodiment, as shown in FIG. 9, the Al composition ratios xand y are changed linearly depending on the thickness. However, the Alcomposition ratio's variation profile may be a quadric, stepwise, or thelike. For example, such composition ratio profiles as exemplified inFIGS. 10-12 can also be expected to achieve the present invention'seffects.

As described in the ninth embodiment, to reduce cracks in the AlGaN cladlayer by changing the Al composition ratio continuously or stepwisebetween the active layer and the transparent substrate having arefractive index higher than the waveguide's effective refractive index,it is important that the clad layers between active layer 707 andsubstrate 701 have a total thickness d_(t) in a range of 1.4μm≦d_(t)≦4.5 μm. It should be noted in this case that the portion ofthickness d, functioning as the clad layer is only a portion having arefractive index smaller than the waveguide's effective refractiveindex. In the case of FIG. 9, for example, a region having the Al ratioof the mixed-crystal exceeding 0.04 is the portion functioning as theclad layer, and has thickness d_(t) of 1.8 μm.

Furthermore, by setting a range of 0.06≦x_(max)≦0.35 for an Alcomposition ratio x_(max) of a portion of the clad layer that has thesmallest refractive index (the highest Al composition ratio) betweenactive layer 707 and substrate 701, it become possible to reduce cracksin the clad layer and effectively reduce the radiation mode intosubstrate 701. Furthermore, by setting the center of the portion havingx_(max) in the clad layer to be at a position upper by more than2d_(t)/3 in the thickness direction from the bottom surface of the cladlayer, the full width at half maximum of the vertical radiation patternin the FFP can be made smaller than 21°.

In the ninth embodiment, the refractive index is continuously changed torealize the function similar to the clad layer formed of threesublayers. In this embodiment, first n-type clad layer 703, the thirdn-type clad layer 705 or the interface therebetween may include aninserted thin layer having a thickness of one fourth or less of thelaser wavelength in the waveguide (in the present embodiment, athickness of 0.04 μm or less) and also having a refractive indexdifferent from those of the overlying and underlying adjacent cladlayers. More specifically, a crack prevention layer of GaN having athickness in a range of 0.005 μm to 0.04 μm may be inserted at theinterface between first and third n-type clad layers 703 and 705, or abuffer layer of GaN having a thickness in a range of 0.01 μm to 0.04 μmmay be inserted in first n-type clad layer 703.

While in the ninth embodiment a graded junction is realized bycontinuously changing the Al composition ratio by way of an example, itis of course possible to obtain the present invention's effects with aclad layer having a superlattice structure formed of many stacked layersincluding GaN and AlGaN layers with their ratio in thickness graduallychanged.

Tenth Embodiment

FIG. 13 is a schematic front view of a laser device provided by a tenthembodiment. This laser device is different from the first embodimentonly in that a portion lower than the first n-type clad layer 103 on thelight emitting end surface is provided with an optical absorber film800. Optical absorber film 800 can prevent light from emanating from thelaser device through underlying GaN layer 102 and GaN substrate 101 andnegatively affecting the FFP. Furthermore, optical absorber film 800 canalso reduce an effect of a radiation mode (a leakage mode) on the FPeven in a semiconductor stacked-layer structure in which the radiationmode still remains more or less, and thus the full width at half maximumof the vertical radiation pattern in the FFP can be further narrowerthan in the first embodiment. Note that optical absorber film 800 may beformed of any metal or resin that can absorb laser light.

Furthermore, as shown in FIG. 14, optical absorber film 800 is formed ina range preferably covering a 35% or more area of a noise lightradiation region 810 below first n-type clad layer 103 on the lightemitting end surface. More preferably, in noise light radiation region810, a portion 820 just underlying the ridged stripe (or the waveguide)is covered by 60% or more with optical absorber film 800. Opticalabsorber film 800 thus arranged can reduce the negative effect of theradiation mode on the FFP. Furthermore, the film can also prevent lightexternally directed to the end surface of substrate 101 from beingreflected by the end surface or entering substrate 101, thereby causingan effect of eliminating unwanted light in an optical system such as anoptical pickup.

The laser device of the tenth embodiment exhibits such an effect forexample in an optical pickup in an optical disk apparatus in which a3-beam method is used for generating a tracking signal. By formingoptical absorber film 800 on a region including positions on the endsurface of substrate 101 which are illuminated with returned sub-beamsof the three beams, it becomes possible to eliminate a tracking signalnoise attributed to interference of the returned sub-beams and theradiation mode light radiated from the laser device's interior throughthe substrate 101 end surface.

Eleventh Embodiment

An eleventh embodiment is different from the tenth embodiment only inthat optical absorber film 800 is replaced with an optical reflectivefilm. In the eleventh embodiment, it is also possible to obtain similareffects as in the tenth embodiment. In the eleventh embodiment, theoptical reflector film should also be formed on the same region asoptical absorber film 800 is formed in the tenth embodiment.

Furthermore, the reflective film can be a metal film or a multilayerdielectric film having a laser light transmittance of 50% at most. Morespecifically, the end surface of the laser device may have a regionprovided with a dielectric film and then Al, Au, Pt, Ni or a similarmetal film having a thickness of 10 nm or more deposited on thedielectric film by evaporation, or with a multi-layer dielectric filmincluding four SiO₂ and TiO₂ layers deposited so as to have areflectance adjusted to 50-70%.

INDUSTRIAL APPLICABILITY

According to the present invention, in a semiconductor laser deviceincluding a transparent substrate having a refractive index larger thanan effective refractive index of its waveguide, it is possible tosignificantly reduce radiation loss into the substrate by selectingmaterial and thickness of a clad layer between the substrate and anactive layer so as to satisfy a prescribed refractive indexrelationship. As a result, it becomes possible to realize asemiconductor laser device having a small threshold current andexcellent reliability. Furthermore, according to the present invention,in a semiconductor laser device, it is possible to effectivelypreventing cracks from occurring in crystals thereby improvingproduction yields and also reducing radiation loss into the substrate.

1. A semiconductor laser device having a waveguide constructed in astack of layers including, on a substrate transparent and having arefractive index n_(s) for laser light, a first clad layer of arefractive index n_(c1), a second clad layer of a refractive indexn_(c2), a third clad layer of a refractive index n_(c3), a firstconductivity type guide layer of a refractive index n_(g), an activequantum well layer, a second conductivity type guide layer, and a secondconductivity type clad layer deposited in this order, wherein saidwaveguide has an effective refractive index n_(e), and a relationship ofn_(c2)<(n_(c1), n_(c3))<n_(e)<(n_(s), n_(g)) is satisfied.
 2. Thesemiconductor laser device of claim 1, wherein said first clad layer hasa thickness of d_(c1), said second clad layer has a thickness of d_(c2)and said third clad layer has a thickness of d_(c3), and then d_(c2),d_(c3)<d_(c1) and 1.4 μm≦d_(c1)+d_(c2)+d_(c3) are satisfied.
 3. Thesemiconductor laser device of claim 1, wherein said second clad layer ismade of a group III-V semiconductor containing Al and said activequantum well layer is made of a group III-V semiconductor containing In.4. The semiconductor laser device of claim 1, wherein said substrate andsaid stack of layers are made of nitride semiconductors, and then saidfirst, second and third clad layers all contain Al.
 5. The semiconductorlaser device of claim 4, wherein said first clad layer has a thicknessof d_(c1), said second clad layer has a thickness of d_(c2) and saidthird clad layer has a thickness of d_(c3), and then d_(c2),d_(c3)<d_(c1) and 1.4 μm≦d_(c1)+d_(c2)+d_(c3)≦4.5 μm are satisfied. 6.The semiconductor laser device of claim 5, wherein said second cladlayer is made of a nitride semiconductor having an Al composition ratiox_(c2) of 0.06≦x_(c2)≦0.3 in the group III elements, and has a thicknessd_(c2) of 0.05 μm≦d_(c2)≦0.35 μm.
 7. The semiconductor laser device ofclaim 6, wherein said first clad layer has an Al composition ratiox_(c1)≦0.07 in the group III elements and said third clad layer has anAl composition ratio x_(c3)≦0.07 in the group III elements.
 8. Thesemiconductor laser device of claim 1, wherein on a laser beam emittingend surface, one of an optical absorber and an optical reflective filmis formed in an optical radiation region below said first clad layer. 9.The semiconductor laser device of claim 8, wherein said optical absorberfilm or said optical reflective film is formed on at least 35% area ofsaid optical radiation region.
 10. The semiconductor laser device ofclaim 8, wherein in said optical radiation region, said optical absorberfilm or said optical reflective film is formed on a portion of at least65% of an area located below said waveguide.
 11. The semiconductor laserdevice of claim 8, wherein said optical reflective film has an opticaltransmittance of at most 50%.
 12. A nitride semiconductor laser devicehaving a waveguide constructed in a stack of layers including, on asubstrate transparent and having a refractive index n_(s) for laserlight, a first conductivity type clad layer, a first conductivity typeguide layer of a refractive index n_(g), an active quantum well layer, asecond conductivity type guide layer, and a second conductivity typeclad layer deposited in this order, wherein: said optical waveguide hasan effective refractive index n_(e)<n_(s), n_(g), said firstconductivity type clad layer includes a first region, a second region,and a third region in this order in its thickness direction, said secondregion having an Al composition ratio larger than said first and thirdregions, and then said first, second and third regions all having theirrespective refractive indexes smaller than n_(e).
 13. The nitridesemiconductor laser device of claim 12, wherein a total thickness d_(t)including said first, second and third regions is in a range of 1.4μm≦d_(t)≦4.5 μm.
 14. The nitride semiconductor laser device of claim 13,wherein in said second region, a maximum Al composition ratio x_(max) isin a range of 0.06≦x_(max)≦0.35.
 15. The nitride semiconductor laserdevice of claim 13, wherein a portion having the maximum Al compositionratio x_(max) in said second region in said first conductivity type cladlayer is located at a position farther than 2d_(t)/3 in a direction fromsaid substrate toward said active layer.
 16. The semiconductor laserdevice of claim 12, wherein on a laser beam emitting end surface, one ofan optical absorber film and an optical reflective film is formed on anoptical radiation region below said first conductivity type clad layer.17. The semiconductor laser device of claim 16, wherein said opticalabsorber film or said optical reflective film is formed on at least 35%area of said optical radiation region.
 18. The semiconductor laserdevice of claim 16, wherein in said optical radiation region, saidoptical absorber film or said optical reflective film is formed on aportion of at least 65% of an area located below said waveguide.
 19. Thesemiconductor laser device of claim 16, wherein said optical reflectivefilm has an optical transmittance of at most 50%.